Natural gas fuels have been found to be one of the most environmentally friendly fuels for use in vehicles and hence the desire by environmental groups and governments to support the use of natural gas in road going applications. Natural gas based fuels are commonly found in three forms; Compressed Natural Gas (CNG), Liquefied Natural Gas (LNG) and a derivative of natural gas, Liquefied Petroleum Gas (LPG).
Natural gas fuelled vehicles have impressive environmental credentials as they emit very low levels of SO2 (sulphur dioxide), soot or other particulate matter, and compared to gasoline and diesel powered vehicles, their emission of CO2 (carbon dioxide) is potentially lower due to a more favourable carbon-hydrogen ratio in the fuel. Natural gas vehicles come in a variety of types, from small cars to (more commonly) small trucks and buses. Natural gas fuels also potentially provide engines with a longer service life and lower maintenance costs. Further, CNG is the least expensive alternative fuel when comparing equal amounts of fuel energy. Still further, natural gas fuels can be combined with other fuels, such as diesel, to provide similar benefits mentioned above.
A key factor limiting the use of natural gas in vehicles is the storage of the natural gas fuel which, in the case of CNG and LNG, the fuel tanks are expensive, large and cumbersome relative to tanks required for conventional liquid fuels with the same energy content. In addition, the lack of availability of CNG and LNG refuelling facilities and the cost of LNG add further limitations on the use of natural gas as a fuel in mobile applications. Further, in the case of LNG, the cost and complexity of producing LNG and the issues associated with storing a cryogenic liquid on a vehicle further limits the potential uptake of this fuel.
This is not quite the same for LPG, which is not a cryogenic liquid, and this fuel is widely used in high mileage motor cars such as taxis. However, the cost benefits are not as clear as in the case of private motor cars and the issues associated with the size and shape of the fuel tank, the cost variability of LPG and the relatively limited supply means that LPG has its limitations also. Consequently, without massive investment in a network of LNG plants around the major transport hubs, CNG is the only feasible form of natural gas that is likely to be widely utilised in the near future.
The method for delivering natural gas into an internal combustion engine can be broadly categorized into two main groups:
Low Pressure Carburetted Induction or Manifold Based Injection:
The practice of inducting natural gas into the inlet of an internal combustion engine is well known and is similar to LPG fuelled vehicles. Because of the ignition characteristics of inspirited natural gas compared to direct injection diesel, the level of liquid fuel substitution when used in a diesel engine using low pressure carburetted induction/manifold based injection is somewhat limited. Another problem with this method is the ‘methane slippage’ that results from the overlap of the inlet and exhaust valves, and/or non-combustion zones in the cylinder chamber typically in the piston-land gap. This results in a level of unburnt hydrocarbons in the engine exhaust that can negate most of the greenhouse gas emission benefits of using natural gas.
High Pressure Direct Injection:
In the case of high pressure direct injection (HPDI), the natural gas is injected into the cylinder with a small quantity of pilot diesel fuel (typically between 3% and 5%) with the result that there is no little potential for methane slippage or pre-ignition of the fuel-air mix. As a result a diesel engine operating on natural gas with high pressure direct injection retains the benefit of the high efficiency of a diesel engine, is able to achieve better than 95% displacement of the liquid fuel, and achieves significant reductions in greenhouse gas emissions and pollutants including sulphur dioxide, carbon dioxide, oxides of nitrogen and soot. Thermal tip ignition or spark ignition are alternatives to diesel pilot ignition and results in a 100 percent gas direct injection engine.
However, HPDI requires the natural gas to be supplied to the engine at a consistent high pressure (typically greater than 3000 psi). For LNG this is achieved through the use of a specially designed pump capable of operating at cryogenic temperatures and delivering the fuel at the required pressure. For CNG it requires an expensive and complex gas compressor that must deliver natural gas at the required pressure from a range of pressures typically between 10 psi (near empty CNG tank) and 3600 psig (full CNG tank). This means the gas compressor set must have the capability to reject the significant quantities of heat created by a compression ratio of up to 300:1 in order to full utilise a tank of CNG. Alternatively a significant amount of fuel is left within the tank to limit the gas compression ratio. This requires large air to gas intercoolers, consumes large quantities of energy and requires a large amount of space which is something not available on most vehicles. While LNG has had some success as a liquid fuel replacement in some regions of the world, the lack of availability of LNG and its high cost means that in many regions of the world it is not feasible to use LNG.
In the case of CNG, it also has had some success as a liquid fuel replacement but almost exclusively in spark ignition engines utilising the low pressure carburetted/port injection induction technology. This application is popular in government bus fleets around the world where the cleaner burning natural fuel is used in a spark ignition engine to replace a conventional diesel engine.
The availability of a system to maintain a high CNG pressure for direct injection means that high horse power CNG has not been considered practical and many in the field have pursued LNG as the only viable natural gas fuel that can be readily pumped/maintained at a high pressure as a liquid to meet the pressure requirements of direct injection.
CNG also has significant issues with transfer of CNG from fixed storage to a vehicle. These issues involve the generation of excessive heat during transfer which limits fill capacity. Further, the fixed storage pressure varies limiting its ability to refuel.